The prior art data transmission system using a server computer is outlined in a block diagram in FIG. 14.
In the prior art system, a plurality of terminal computers 12 are connected to a server computer 11 via switching hub 13 with a packet switching function, and upon demand from a terminal computer 12, the server computer 11 transmits the wanted data to the terminal computer that sent in the demand.
The prior art network data transmitter is illustrated in further detail in FIG. 15. The server computer 11 is provided with k units of cards 130. each comprising a local CPU 131(k) and a local memory 132(k) as well as a port correspoonding to a local disk 133(k); k units of send-only ports Ps1(k), one for each card 130(k); and one receive-only port Pr1. The suffix k in parentheses indicates the cardnumber and is an integer (in the case of FIG. 15, an integer of 0 to 7).
The switching hub 13 is equipped with k units of ports Ps2 (k) as opposed to the aforesaid send-only ports Ps1(k) on the server computer and a port Pr2 opposite to the above-mentioned receive-only port Pr1 also on the server computer 11 as well as a plurality of ports p(a) to p(c) for the terminal computers 12 (in the case of FIG. 15, 3 terminal ports). To each of the terminal ports p(a) to p(c) can be connected one or more terminal computers 12. In FIG. 15, three units of terminal computers 12-1 to 12-3 are connected to the terminal port p(a), and one terminal computer 12-4 to the terminal port p(b) and one 12-5 to the terminal port p(c) respectively.
The local disks 133(k) on the aforesaid server computer combined forms a logical storage area. Physically, one data is fragmented in blocks of, say one kilobyte, and stored there.
FIG. 16 is a functional block diagram showing the configuration of the server computer in still more detail. The operating process of the prior art system will now be described further elaborately, referring to the figure.
If the terminal computer 12-1 sends in a data processing demand packet Pd1 as shown in FIG. 17(a), for example, the packet is transmitted to the server computer 11 (the receive only port Pr1) via the terminal port p(a) and the port Pr2 on the switching hub 13. By the way, the data processing demand packet Pd1 contains the following information as shown in FIG. 17(a): address (server computer address) (1), sender's address (address of the terminal computer making a data processing demand) (2), data reading out demand (3), file number (4), address of data to be read out (address on the logical storage area formed by the whole local disks) (5) and data size (6). The data processing demand packet Pd0 which is received at a packet receiver 101 in the server computer is relayed to a packet processor 102 where the processing demand Pd1 is deciphered and sent on to a data processor 103.
The data processor 103 in turn instructs a data reader 105 to read out data at a specified address, the reader provided on the local CPU 131(k) in each card 130(k). According to the instructions, the data reader 105 reads out the specified data on to a local memory 132(k) from the local disk 133(k) provided in the card 130(k).
Further, the data processor 103 hands over the "sending" demand packet Ps1(k) shown in FIG. 17(b) to a sending data packet processor 104 in the card 130(k), and instructs the processor to incorporate the read out data into a packet. A group of sending data packets thus formed in the card 130(k) is sent out from the packet sender 106 via the port Ps1(k) provided for the card 130(k). It is noted that the "sending" demand packet Ps1(k) carries the following information as shown in FIG. 17(b): destination address (address of the terminal computer) (1), sender's address (address of the server computer 11) (2), packet number (7), address of the data to be read out (address of the local disk for the data to read out) (8), and data size (6).
The sending data packet thus sent out is outputted to the switching hub 13, where the packet is stored in the buffer of the port P(a) for the destination terminal computer 12-1 before being send out therefrom. The data packet outputted this way has the address written at the header, and can be taken in by the terminal computer 12-1 at that address. Receiving a group of sending data packets, the terminal computer 12-1 edits the packets of data in a proper order and uses them for the intended purpose.
The question is that there is a possibility that the size of a single sending data packet will exceed the maximum permissible volume of a packet that flows on the network. To cope with it, a plurality of data packets, say 8 sending data packets, are usually grouped into one unit. In the ethernet, the maximum permissible packet size is 1.5 kilobytes for single packets. But if a plurality of data packets are grouped into a unit, up to 8 kilobytes can be transmitted as one unit. In an example with 8 local disks, a unit of 8 kilobytes of data packets can be formed with one kilobyte allocated for each local disk 133(k).
In the foregoing system configuration, however, there arises a possibility that a plurality of sending data packet units or groups may be transmitted almost simultaneously and successively to a specific terminal port on the switching hub to which the terminal computers 12 are connected.
When processing demands happen to come almost simultaneously from terminal computers 12-1 and 12-2 both connected to the port P(a) on the switching hub 13, for example, two units of sending data packets to be received by the terminal computers 12-1 and 12-2 [(the number of packets in a send packet group).times.2] will rush to the port P(a) on the switching hub 13 almost at the same time.
Also, when data processing demand packets Pd0 are sent almost simultaneously from three computers 12-1 to 12-3, for example, the switching hub 13 will have deal with the same number of sending data packets [(number of packets in a send packet group).times.3] the same way.
While a buffer is provided in the terminal port p(a) on the switching hub 13, a simultaneous rush of data could exceed the capacity of the buffer. If too large a size of data rushes in the buffer, the part of the data that fails to be stored in the buffer will be discarded. That is, the units of data sent out from the server computer 11 will come in with the data for the terminal computer 12-1 or 12-2 cut off in part. That presents such problems as interrupted moving images as in a moving image data system, for example, where real time processing is required.
The present invention is proposed in view of such problems that are encountered with the prior art system. It is an object of the present invention to provide a server computer so configured that if there are simultaneous processing demands from a plurality of terminal computers connected to the switching hub, send data would not be concentrated in one terminal port on the switching hub.